Download 15. Scientific Realism and Scientific Explanation 1. Scientific Realism

15. Scientific Realism and Scientific Explanation
1. Scientific Realism
• What does the world consist of?
- electrons, chemical elements, genes, etc.
• Was the world of one thousand years ago a
world of electrons, genes, etc.?
- Scientific realist: Yes, although nobody knew it
back then.
- Scientific anti-realist: No, the concepts of electron, gene,
etc., were the product of debates and experiments in
specific historical contexts (contingencies). These
concepts are subject to change in future theories.
A. Types of Realism
Common-sense Realism: We inhabit a common reality, which
has a structure that exists independently of what people think
and say about it, except insofar as reality is comprised of, or
is causally effected by, thoughts, theories, and other symbols.
• Could science tell us that common-sense realism is false?
Ex. 1: Quantum mechanics.
• Two ways a QM state can change:
1. In absence of measurement, a state changes via the Schrödinger equation.
|ψ(t1)〉 !!!→ |ψ(t2)〉
Schrödinger
evolution
2. In presence of measurement, states change via the Projection Postulate:
When a measurement of property B is made on a state |ψ〉 = a1|b1〉 + ... + aN|bN〉
expanded in the eigenvector basis of B with result bi , then |ψ〉 collapses to |bi 〉:
|ψ〉 !!!→ |bi 〉
collapse
• What is a measurement? When is the Projection Postulate
supposed to take over from the Schrödinger dynamics?
- When a conscious observer looks at a measuring device?
Eugene Wigner
- When a macroscopic system interacts with a microscopic system?
- There is no Projection Postulate: when a measurement occurs,
the world splits into as many duplicate worlds as there are
possible outcomes of the measurement.
Hugh Everett
A. Types of Realism
Common-sense Realism: We inhabit a common reality, which
has a structure that exists independently of what people think
and say about it, except insofar as reality is comprised of, or
is causally effected by, thoughts, theories, and other symbols.
• Could science tell us that common-sense realism is false?
Common-sense Realism Naturalized: We inhabit a common reality,
which has a structure that exists independently of what people
think and say about it, except insofar as reality is comprised of, or
is causally effected by, thoughts, theories, and other symbols, and
except insofar as reality is dependent on thoughts, theories, and
other symbols in ways that might be uncovered by science.
Scientific Realism:
1. Common-sense realism naturalized.
2. One actual and reasonable aim of science is to give us accurate
descriptions (and other representations) of what reality is like,
including aspects of reality that are unobservable.
• Optimistic Scientific Realism: We can be confident that science is successful
in this aim.
• Pessimistic Scientific Realism: We can hope that science is successful in this
aim, although it is very difficult.
- Ex: Kuhn (pessimistic scientific realist): We try to 'force' nature into 'boxes' but
nature resists. All paradigms are doomed to fail eventually.
Pessimistic Meta-Induction: All theories in the history of science have
been wrong, so current and future theories must and will be wrong, too.
Past theories now thought wrong:
- Aristotle's theory of motion.
- Humoral theory of disease.
- Caloric theory of heat.
- Phlogiston theory of chemical reactions.
- Aether theories of optics and electromagnetism.
- Vital force theories in physiology.
- Newton's theory of motion.
• Basic Issue: What level of confidence should we have in our current theories?
Miracle Argument: Realism is the only philosophy of
science that does not make the success of science a miracle.
• But: There are many kinds of ways in which the link between theory and
reality can generate success.
- Accurate representation of the world is not the only way.
Ex. 2: Carnot, S. (1824) "Reflections on the Motive Power of Fire"
• Idea: Treat heat in analogy with water as a substance
("caloric") that produces mechanical effect (work)
when it "falls" from a hot place to a cold place.
Sadi Carnot
1796-1832
hot place
⇓
heat in
heat
engine
⇓
⇒ work out
heat out
cold place
• Important question: What is the maximally efficient heat engine?
Maximum efficiency is obtained when heat-flow between hot place
and engine, and engine and cold place, occurs at equal temperatures.
• Analogy with water-wheel: Maximum efficiency obtained when water-flow
between stream and water-wheel occurs at equal heights (minimizes splashing).
Claim: The maximum efficiency of any heat engine is equal to that of a
reversible heat engine operating between the same hot and cold places.
Proof:
• Suppose we have a reversible engine A that produces work W.
• Suppose there is a more efficient engine B between the same hot and
cold places (B uses the same heat as A and produces more work).
• Now reverse A and hook it up to B.
hot place
hot place
⇓
⇓
Q
A
(REV)
⇓
Q
cold place
⇒W
Q
⇒W + w
B
⇓
Q
cold place
Claim: The maximum efficiency of any heat engine is equal to that of a
reversible heat engine operating between the same hot and cold places.
Proof:
• Suppose we have a reversible engine A that produces work W.
• Suppose there is a more efficient engine B between the same hot and
cold places (B uses the same heat as A and produces more work).
• Now reverse A and hook it up to B.
hot place
hot place
⇑
⇓
Q
A
(REV)
⇑
Q
cold place
⇐W
Q
⇒W + w
B
⇓
Q
cold place
Claim: The maximum efficiency of any heat engine is equal to that of a
reversible heat engine operating between the same hot and cold places.
Proof:
• Suppose we have a reversible engine A that produces work W.
• Suppose there is a more efficient engine B between the same hot and
cold places (B uses the same heat as A and produces more work).
• Now reverse A and hook it up to B.
• Engine (A+B) does work w for free (no net fall of heat required)!
But, sez Carnot, this is impossible: a perpetual motion machine!
hot place
⇓
Q
⇓
A
⇒
(REV)
hot place
Q
⇒
B
W
Q
cold place
w
⇓
=
A+B
Q
cold place
⇒
w
⇓
Ex. 3: Maxwell, J. (1861) "On Physical Lines of Force"
• Idea: Treat electromagnetism as a mechanical effect
of an "aether" made up of vortices and idle wheels.
(a) Ampère's Law: J = ∇ × H. Current density (J) in a wire
generates a magnetic intensity (H) around the wire.
James Clerk Maxwell
(1831-1879)
(b) Faraday's Law: −∂µH/∂t = ∇ × E. A changing magnetic intensity (−∂µH/∂t )
through a wire loop generates an electric field (E), around the loop.
Suppose the aether consists of a (3-dim) array of
spinning "vortex" cells separated by moving idle wheels:
Purely mechanical results:
(a) (flux of idle-wheels) = ∇ × (angular
velocity of vortices).
(b) (change in angular momentum of
vortices) = ∇ × (tangential force of
idle-wheels on vortices)
Let J = flux of idle-wheels
H = angular velocity of vortices
µ = density of aether
E = tangential force of idle-wheels on vortices
B. Objections to Realism
1. Traditional Empiricism
Underdetermination: There will always be a range of
alternative theories compatible with all possible evidence; thus
we never have good empirical grounds for choosing one over the
others and regarding it as representing how the world really is.
Underdetermination of interpretations of a single theory:
• Should evidence for general relativity count as evidence for
- the existence of spacetime as a real substance independent of physical objects?
- the existence of spacetime as certain relations between physical objects?
• Should evidence for quantum mechanics count as evidence for
-
observer-dependent measurement outcomes?
many worlds?
many minds?
etc.
B. Objections to Realism
1. Traditional Empiricism
Underdetermination: There will always be a range of
alternative theories compatible with all possible evidence; thus
we never have good empirical grounds for choosing one over the
others and regarding it as representing how the world really is.
Underdetermination of theories by evidence:
• Should evidence for Newton's law of gravity count as evidence for
- Newtonian gravity as a force in flat Neo-Newtonian spacetime?
- Newtonain gravity as an effect of the curvature of Newton-Cartan spacetime?
• Should (fledgling) evidence for quantum gravity effects count as evidence for
-
canoncial quantum gravity?
string theory?
causal set theory?
etc.
2. Metaphysical Constructivism
Metaphysical Constructivism: The world, in some
sense, is created or constructed by scientific theorizing.
• Kuhn, Strong Programme, ANT.
• Recall: Modified versions of Kant?
- Noumenal world ("raw data") versus phenomenal world ("filtered data").
- Kant's claim: Filters (categories) are universal and absolute: There's only one way
we construct the phenomenal world from the noumenal world.
- Kuhn/Strong Programme/ANT: Filters are the products of culture and society and
are not absolute and universal.
• Objection to realism: "There is a real world (the noumenal world) constraining
what we believe but in a way that does not permit our knowing or
representing this world."
Problem: Is this really incompatible with (pessimistic) scientific realism?
3. Constructive Empiricism
• An empirically adequate theory accurately describes the
observable parts of the world.
• A true theory accurately describes both the observable and
unobservable parts of the world.
Van Frassen, B.
(1980) The
Scientific Image
Constructive Empiricism: The aim of science is to provide empirically
adequate theories. To accept a theory is to
1. believe (provisionally) that the theory is empirically adequate, and
2. use the concepts the theory provides when thinking about further
problems and when trying to extend and refine the theory.
• So: A theory can be accepted while remaining agnostic about its truth.
• And: Why risk anything more epistemically?
Problem: How is the distinction between observable
and unobservable parts of the world to be made?
- Realist: "...there is a continuum, rather than a sharp
distinction, between the observable and the unobservable".
2. Scientific Explanation
Instrumentalism: Scientific theories are devices for
helping us deal with experience. They are instruments
("black boxes") used to make and test predictions.
• But: Isn't there more to science than prediction? Doesn't science aim at
explaining phenomena?
• What is it for a scientific theory to explain something?
Terminology:
- Expanandum = whatever is being explained.
- Explanans = the thing that is doing the explaining.
A. Covering Law Account
Covering Law Account: To explain something is to show how to derive
it in a logical argument that makes use of a law of nature, such that:
(a)
(b)
(c)
(d)
(e)
The
The
The
The
The
type
conclusion is the explanandum.
premises are the explanans.
premises contain at least one statement of a law of nature.
premises are true.
argument can be either deductive or inductive (when deductive, this
of explanation is called "deductive-nomological")
explanans
L1, L2, ...
C1, C2, ...
law(s)
explanandum
∴ O1, O2, ...
observed phenomena
conditions underwhich laws are applicable
• Nomic expectability: To explain something is to show that it is nomically
(lawfully) expected.
Ex. 1: Why does an ice skater spin faster as he brings his arms in towards his
body?
1.
2.
3.
4.
Angular momentum is conserved.
Skater doesn't interact with external objects.
Skater has non-zero initial angular momentum.
Skater brings arms in towards body.
(Li = Lf)
(Li = Ii ωi ; Lf = If ωf )
(Li ≠ 0)
(If < Ii)
law
∴
Skater spins faster.
(ωi > ωf)
observed
phenomenon
conditions
• Note: If we only knew the law and conditions in the premises, we could
predict the observed phenomenon.
Symmetry thesis: Every covering law explanation is a potential
prediction. Every prediction is a potential covering law explanation.
Problem: Not every prediction is a potential scientific explanation.
Ex. 2: Why does the shadow of this flagpole have length l ?
1.
2.
3.
4.
Light propagates linearly.
Sun is at certain elevation.
<Relevant trigonometic relations>.
Flagpole has height h .
∴
Shadow has length l .
law
conditions
observed phenomenon
• Now: Consider the following prediction of the height of a flagpole, given it's
shadow has length l and various conditions hold:
1.
2.
3.
4.
Light propagates linearly.
Sun is at certain elevation.
<Relevant trigonometic relations>.
Shadow has length l .
law
∴
Flagpole has height h .
observed phenomenon
conditions
• Does this prediction count as a scientific explanation of the flagpole's height?
- Why does this flagpole have height h? Because it's shadow has length l?
B. Causal Account
• Intuition: The flagpole caused the shadow, and thus can explain it. The
shadow did not cause the flagpole, and thus cannot explain it.
Causal Account: To explain something is to describe what caused it.
Ex. 3: Why do automatic sliding doors open when you approach them?
- Because when you approach an automatic sliding door, you step into
the beam of an optical sensor, and this causes a circuit to be broken,
which subsequently causes a mechanism to open the door.
Problems:
• Causation is not entirely understood.
• Purely theoretical explanations occur in science.
Ex. 4: Why can't you fit a left-handed glove on your right hand?
- Because a left-handed glove and your right-hand are topologically
inequivalent in 3-dim Euclidean space (they are enantiomorphs).
C. Unification Account
• Intuition: To explain is to connect a diverse set of facts by subsuming them
under a set of basic patterns and principles.
Unification Account: To explain something is to demonstrate
how it belongs to the most unifying systematization of the set
of claims currently endorsed by the scientific community.
• A systematization Σ of a set of claims is a subset of those claims from which
the rest can be derived.
• Σ is unifying if it maximizes scope, simplicity and stringency.
- Scope measures the number of conclusions that can be drawn from Σ.
- Simplicity measures the size of Σ.
- Stringency measures the range of applicability of Σ.
Ex. 5: Unification as a goal in physics.
- Maxwell's theory of electromagnetism provides unifying
explanations of electric and magnetic phenomena.
- The Standard Model provides unifying explanations of phenomena
that experience the electromagnetic, strong, and weak forces.
D. Contextual Pluralism
Contextual Pluralist: Science aims at explaning phenomena, but the
standards of a good explanation depend on the scientific context.
"It is a mistake to think there is one basic relation that is
the explanatory relation..." (Godfrey-Smith 2003, pg. 197.)
• Standards of explanation vary between scientific disciplines at any given time,
and within disciplines at different times.
Ex. 6: Synchronic and diachronic contextual pluralism
- Synchronic: Explanations in biology (causes/mechanisms)
versus explanations in theoretical physics (unifying).
- Diachronic: Aristotelian explanations (occult forms) versus
Cartesian explanations (causes/mechanisms).